1. Field of the Invention
This invention relates in general to DC motor starting circuits and more particularly to a DC motor starting circuit with a current limiter.
2. Background Information
When DC motors are used for valve operators, the high starting current can place undue burden on the electrical cables and a battery power source. The current in a DC motor is limited by the self generated electromotive force (EMF) voltage that is proportional to the machine speed. When the motor is first started, the speed and hence the EMF is zero (0), thus allowing the current to rise to the rated value known as the “locked rotor current”, which can be up to ten times the name plate current rating of the motor. As the machine accelerates, the EMF increases and the current decreases until a steady equilibrium is achieved depending on the load. FIG. 1 shows the current and speed as a function of time for a typical 15 horsepower motor. The current curve is represented by reference character 10 and the speed curve is represented by reference character 12.
Conventional reduce voltage motor starters limit the current but do not adapt to changing conditions of motor temperature or supply voltage. In a conventional reduced voltage motor starter, additional resistance is included in the circuit to limit the current and is then switched out as the motor accelerates. This may be done as a single step or in multiple steps. The switching may be based on fixed times or on a measurement of the motor speed (actually the EMF developed across the motor). A circuit for implementing this approach is illustrated in FIG. 2A. R and L respectively stand for the resistance and inductance of the motor windings. EMF is the electro-motive force developed within the motor. Resistance R1 is greater than R2. In operation the Switch S is sequenced from OFF to R1. Then, after a preselected time interval has passed or motor speed has been achieved the Switch S is sequenced to R2 and then when the motor reaches full speed the Switch S connects directly to the power supply. This approach has several drawbacks. In the switched resistance approach, the current limiting is only provided at initial start up. If the current increases later due to overload conditions, then the current will increase until it is interrupted by an over-temperature protection device, if one is included in the circuit. Furthermore, the resistors must be sized to dissipate a large amount of heat. For example, in the 15 horsepower model used to produce the graphs shown in the figures, the RMS power dissipation of the first stage resister is 6.9 KW. Additionally, once the starting resistances have been chosen, it is difficult to make changes in the field to accommodate the as installed conditions experienced for each specific motor operated valve.
The advent of high power semiconductors allows a switch mode voltage regulator approach employed by the prior art to control the voltage of a DC motor as a means to control the motor's speed, to be adapted to overcome the issues identified above for the conventional reduced current motor starter. This is particularly important for some critical applications such as, advance passive nuclear power plants, where DC motors will be employed for safety grade valve operators because no class 1 E AC power source is available. Because these motors must operate from the 125 VDC power station batteries, it is not possible to increase motor voltage for the larger size motors to decrease the operating current. The advanced reactor Utilities Requirements Document for these passive nuclear power plants requires that the cables connecting the valve operators be sized to carry the full locked rotor current on a continuing basis. This is due, in part to a United States Nuclear Regulatory Commission Regulatory Guide (RG1.106) that requires overcurrent protective devices to be bypassed in the event of a safety actuation of the valves. As mentioned above, conventional relay based motor starters have used reduced voltage means to limit starting current by switching resistors into the circuit upon initial energization This conventional approach has the drawbacks that were mentioned above that need to be overcome.
Accordingly, it is an object of this invention to provide a current limiting DC motor starter that employs a switching mode current regulator in a feedback control loop to limit the motor current to a preset value.
It is a further object of this invention to provide an improved current limiting DC motor starter that integrates a switching mode current regulator with a solid state reversing motor starter.
It is an additional object of this invention to provide a current limiting DC motor starter that employs a switching mode current regulator employing closed loop current measurement to provide precise current control that adapts to changing conditions.
It is the further object of this invention to provide an improved current limiting DC motor starter that provides an overcurrent indication to a control system as well as features to enable its calibration.
Further, it is the object of this invention to provide an improved current limiting DC motor starter having an interface with a control system that is arranged in such a manner that cable faults and other single failures will not cause spurious actuation of the motor.
It is an additional object of this invention to provide such an improved current limiting DC motor starter that is specifically suited for safety grade applications in nuclear power plants.